Solar Energy Collection System

ABSTRACT

A method of concentrating directional radiant energy using reflective optics and receivers that convert that energy wherein the receivers are situated in the body of the reflector on risers parallel to the direction of radiant energy, each said riser bounded by at least one parabolic mirror lying closer and another lying farther from the energy source, where the focus or foci of said mirrors lie substantially in the direction faced by the receiver situated in said riser. The reflector geometries include ones in which the mirrors are parabolic cylinder sections and require only one-axis tacking to focus, and ones in which the mirrors are paraboloid sections and require two-axis tracking to focus sunlight.

PRIORITY Claim of Priority Under 35 U.S.C. §119

The present application claims benefit of U.S. Provisional PatentApplication having Ser. No. 61/131,268, filed Jun. 7, 2008, and U.S.Provisional Patent Application having Ser. No. 61/132,550, filed Jun.20, 2008, wherein the entirety of both of said provisional applicationsare incorporated herein.

FIELD OF THE INVENTION

The present invention relates to methods of collecting solar energyusing optical concentration with moving modules that track the sun, and,in particular, such methods which are suitable for the construction ofsystems having form factors and installation features similar toconventional solar panels.

BACKGROUND

Whereas the optical concentration of directional light onto photovoltaicand/or thermal receivers has long been used in industrial-scale solarpower installations, small-scale solar installations have consistedalmost entirely of plate-type photovoltaic panels without opticalconcentration. Such panels are expensive because of the large quantitiesof photovoltaic material they use, and typically require several yearsof operation just to recover the energy of production of thosematerials. A solar panel that used optical concentration withphotovoltaics (CPV) could potentially provide a much less expensivealternative to plate-type solar panels by greatly reducing thequantities of photovoltaic materials used. Furthermore, a CPV panelmight provide a significantly higher efficiency than a comparableplate-type panel by allowing the economic use of special high-efficiencyphotovoltaic cells whose cost in a panel without concentration would beprohibitive.

If a CPV panel is to be mounted to track the sun, then its design isstraightforward because the sun will remain aligned with the panel'snormal axis. Given the expense and profile that tracking equipment addto solar panel installations, it would be desirable to have a CPV panelthat could function in a fixed-position installation and thereby becomea viable replacement for the common plate-type solar panels. Thecreation of such a panel presents a new set of challenges due to themovement of the sun combined with the form-factor constrains of a solarpanel. An obvious approach is to fill a shallow enclosure with an arrayof concentrating elements, using reflective and/or refractive optics,where each element pivots about its own axis or axes to track the sun.Two key performance metrics of such an approach are the fraction oflight falling on the panel's face that is captured (its apertureefficiency), and the range of tracking motion of the elements. Obviouslya panel that captured 100 percent of the light falling on it and trackedthe sun to incidence angles of up to 90 degrees would be the mostdesirable, but there are numerous issues in existing and proposeddesigns that impact such performance attributes, and many of theseissues arise from constraints imposed by the optical geometry of the CPVelements. The present invention provides a novel reflective opticalgeometry that solves problems in the design of efficient CPV panels.

CPV systems can be characterized by their concentration ratio, expressedas the number of suns falling on their receivers. Systems withconcentration ratios greater than about three generally require the useof moving optical focusing elements that track the movement of the sunacross the sky. Such optical tracking systems fall into two main types:systems in which elongate elements with uniform cross-sectional profilesindividually tilt about their long axis to track the sun and keep itslight focused onto narrow bands, and systems in which elements withradial symmetry individually or as clusters pivot about two axes totrack the sun and keep its light focused onto small spots. The size ofthe disk of the sun, appearing about one-half degree in diameter, limitsthe theoretical concentration ratio achievable with one-axis andtwo-axis systems to a few hundred and a few tens of thousandsrespectively, with practical limits being considerably less due toimperfections in optics and tracking.

The terms one-axis and two-axis are used to denote these two approaches,referring to the number of tilt axes required to keep the sunlightfocused. The present invention is applicable to both approaches, andexemplary embodiments falling under both approaches are disclosedherein.

The invention is suitable for systems using photovoltaic, thermal, andhybrid receivers over a variety of scales and a range of concentrationratios. However, because the invention allows the creation of systemshaving attributes of system efficiency, heat dissipation, andcompactness that are particularly suited to the application area of CPVpanels, comparisons with prior art made herein focus on attributesrelevant to the performance of close-packed arrays of CPV elements.

This review focuses on prior art in that application area of CPV panels.In particular, it examines one- and two-axis tracking systems thatemploy multiple CPV elements, using primarily reflective optics, eachmounted to tilt about its individual axis or axes, and eachincorporating an optical focusing means and photovoltaic receiver. Anexample is described and its drawbacks characterized for each of threesuch types of such systems: ones in which elements are slats withasymmetric profiles, ones in which the elements are troughs withsymmetric profiles, and ones in which the elements are dishes.

U.S. patent application Ser. No. 12/156,189 describes an array ofpivotably-mounted slats, each mounting a photovoltaic strip straddlingthe focal line of a parabolic cylinder mirror on the facing side of thesame slat.

Since the focal line of a parabolic mirror is separated from points onthe mirror's surface by a distance of at least the parabolic cylinder'sfocal length, the slat must contain a riser extending above thecylinder's surface to support the strip of photovoltaic material. If themirror portion of the slat is to be used as a heat sink to wick heatfrom the photovoltaic strip, then the riser must conduct heat to themirror as well as provide the structural function of rigidly mountingthe strip relative to the mirror, imposing a cost in materials and spacerequirements.

The same invention also has a limitation in the range of angles throughwhich the slats can rotate, and hence the range of angles of directionalincident light projected into a plane perpendicular to the slats' axesthrough which the system can capture that light and operate, that rangebeing from near 90 degrees clockwise of the normal direction to onlybetween about 10 to 30 degrees counterclockwise of it for most practicalvariants. This limitation in the system's coverage of directions ofincident light constrains the choices for its optimal siting, possiblysacrificing coverage for parts of the diurnal and annual cycles.

U.S. patent application Ser. No. 11/654,256 describes a panel having aseries of elongate modules mounted within a frame to pivot about theirindividual axes. Each module has a strip of photovoltaic cells locatedalong its bottom, symmetrically disposed reflectors forming its sides,and a transparent cover with a central lens forming its top, such thatparallel light entering a properly tilted module will either passthrough the non-lens portion of the cover and be reflected from the sidewall to the photovoltaic strip, or pass through the lens portion of thecover and be refracted to the photovoltaic strip.

Because of the geometric constraints imposed by this optical system, themodules' height-to-width ratio is greater than one, and the bottom ofthe trough is displaced from the mid-line of the module'saperture-defining transparent cover by a distance of about three timesthat of either edge of the cover. As a consequence, for the modules tohave any appreciable range of motion, they must be spaced such that aportion of directional light perpendicular to the panel falls betweenmodules, decreasing the panel's effective aperture. Increasing themodules' range of motion increases the required spacing interval,further increasing aperture losses. To accommodate a range of motion of70 degrees to either side of the panel's perpendicular direction entailsan aperture loss of up to about 50 percent.

U.S. patent application Ser. No. 11/454,441 describes a panel in whicheach of an array of CPV elements is pivotably mounted in a basestructure and articulated to a moving bracket that forces the elementsto move in unison about two axes to track the sun. The body of each CPVelement is formed primarily from two halves, each half comprising aparaboloid reflector portion and flat, vertical portions, where thereflector faces of the two halves belong to the same paraboloid, whosefocus is situated between the halves' vertical portions.

Details of the mechanical linkages that articulate the elements to themoving bracket and frame are not well specified, nor is the range ofangular motion of the elements given the linkage. It appears that aspace between each element's said halves enables a significant range ofmotion along one axis, but at the expense of a substantial loss ofaperture. The design uses the vertical portions of the CPV element'shalves both to support its photovoltaic cell, and provide surfaces forheat dissipation. Although thin in profile, these features in the“optical volume” of the element further reduce its aperture. Still moreloss of aperture results from the fact that light reflected fromportions of the reflector near the bases of the vertical portions isblocked from reaching the cell by that portion.

Apart from these issues, the design suffers from a trade-off between thereceiver incidence angles and the CPV elements' tracking range that isshared by other designs that mount a receiver in the space above aparaboloid dish. In order for the CPV element to have a wide range ofangular motion, the structure supporting the receiver must be relativelyshort so as not to collide with adjacent elements, resulting in a highaverage incidence angle of reflected light on the receiver. Because theefficiency of light capture by most photovoltaic cells begins to falloff for incidence angles of more than about 45 degrees, the fact that asignificant fraction of the light captured by CPV elements of said typereaches the receiver at incidence angles in the vicinity of 45 degreesrepresents a potentially significant detriment to the system'sefficiency.

Most two-axis CPV panel designs employ refractive instead of reflectiveoptics. Refractive optics used for solar concentration, however, has anumber of disadvantageous characteristics, including the spreading offocal spots due to chromatic aberration, the susceptibility of opticalplastics to UV degradation and the weight of optical glasses, and, aswith paraboloid dishes, high average incidence angles on receivers fordesigns in which the modules are sufficiently compact to have a widerange of angular motion.

OVERVIEW OF THE INVENTION

The present invention provides numerous solutions helpful singly or incombination to overcome problems inherent in prior art single-axis andmultiple-axis solar concentrators and arrays thereof, including thefollowing. First, the invention allows densely-packed arrays of elementsto individually track through the entire range of angles up to 90degrees away from the normal direction of the array about each axis onwhich the elements are mounted to tilt. Second, the invention allowscapture of virtually all the sunlight falling on the panel's facethrough this entire angular range, eliminating the coverage gaps of manyprior-art designs. Third, the invention provides superior economy ofmaterials by combining the functions of heat sinkage, structuralsupport, and light reflection in a single part, by positioning energyreceivers between adjacent mirror sections.

Like prior art arrangements described above, applications of theinvention to solar energy concentration use arrays of CPV elements eachhaving a reflector, an energy capture device such as a photovoltaiccell, and an integral heat sink. Unlike the prior art arrangements, CPVelements of the invention embed PV cells within spans of the reflectorbody that face, at roughly right angles, other such spans. In thesimplest case, a first cell is positioned between mirrors whose foci lieon a second cell, which is positioned between mirrors whose foci lie onthe first cell. CPV elements based on the invention can have a compactshape that fits within a half-cylinder or a half-sphere whose diameterequals that of the element's aperture, and yet provide a low averageangle of incidence of light on the PV cells.

In the two embodiments described in the most detail herein, the elementsare elongate slats that act as heat sinks for the PV devices, are eachpivotably mounted to tilt about their respective axes, and are equippedwith means to sense the position of the sun and adjust their tiltangles, singly or in unison, so as to maintain the focus of incidentlight along the parabolic surfaces' foci and thus convergent upon theenergy capture devices.

Although most of the embodiments described herein are designed for theapplication of concentrating solar energy, the invention has obviousapplications in other fields. For example, forms of the invention usingparaboloid mirrors may be used in imaging applications, where thereceivers situated at the paraboloids' foci are imaging devices insteadof energy conversion devices.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the accompanying drawings in which like referencenumerals refer to similar elements.

FIG. 1: Examples

FIG. 1 shows six different examples of the invention. FIGS. 1A and Bshow reflectors requiring one-axis tracking, and FIGS. 1C through F showreflectors requiring two-axis tracking to focus sunlight on theirreceivers.

FIGS. 2 through 5: Method Overview

FIG. 2 shows a cross-section of a single slat and portions of itsneighbors, where the slats are oriented so as to absorb directionalincident sunlight.

FIG. 3 shows cross-sections of six one-axis concentrating designs,comparing the optical geometries of five representative examples ofprior art with a that of the simplest form of invention.

FIG. 4 illustrates a method of deriving the profile of a reflector. FIG.4A shows the derivation of the parabolic profiles of one half of areflector, and FIG. 4B shows a complete reflector profile.

FIG. 5 shows cross-sections of six different reflectors, illustratingthe effect of the two principal parameters used in the shape derivationdescribed with reference to FIG. 4.

FIGS. 6 through 13: Rooftop Concentrating Photovoltaic Panel

FIG. 6 shows views of the front exterior of a concentrating solar panelembodiment suitable for installation on rooftops.

FIG. 7 shows details of an assembled and an exploded slat, and across-section of the slat.

FIG. 8 shows the panel in a state of partial dis-assembly.

FIG. 9 shows a view the underside of the panel in which a portion of theback wall has been cut away to reveal the drive mechanism.

FIG. 10 shows a view of the underside of the panel in which a portion ofthe back wall has been cut away to reveal electrical cabling

FIG. 11 shows an exploded view of a panel, in which all of the majorcomponents of the enclosure are separated.

FIG. 12 shows details of a slat's photovoltaic strips.

FIG. 13 shows an electrical schematic of the panel.

FIG. 14: Prototype Concentrating Photovoltaic Panel

FIG. 14 shows an assembled and an exploded slat, and a cross-section ofthe slat.

FIGS. 15 and 16: Asymmetric Slat Profiles

FIG. 15 shows the profile of a slat having a single receiver strip and asecondary mirror opposite that strip.

FIG. 16 shows the profile of a slat with an asymmetric arrangement ofsix parabolic cylinders and four risers.

FIGS. 17 through 26: Window with Retracting Photovoltaic Concentrator

FIG. 17 shows views of the multi-function window embodiment in itsenergy collecting mode, with its internal photovoltaic concentratordeployed and tracking.

FIG. 18 shows views of the embodiment in its transparent window mode,with its concentrator retracted.

FIG. 19 shows a cross-section of a slat, illustrating its opticalgeometry.

FIG. 20 shows cross-sections of a pair of adjacent slats in an array ineach of the embodiment's three operating modes.

FIG. 21 shows assembled and exploded views of a slat and its hangers.

FIG. 22 shows an assembly of four adjacent slats and their hangers,where the slats are in the stacking orientation and the hanger strapsare relaxed.

FIG. 23 shows the assembly shown in FIG. 21, where the slats are intheir shuttering orientation with the hanger straps fully extended.

FIG. 24 shows a cross-section of the embodiment in the retractedposition.

FIG. 25 shows a cross-section of the embodiment in the deployedposition, with the optical axes of the slats oriented 30 degrees abovethe window's normal axis.

FIG. 26 shows the retracting concentrator system revealed by hiding thewindow's exterior frame and glazing.

FIGS. 27 and 28: Two-Axis Reflector Examples

FIG. 27 shows an example of a two-axis embodiment of the invention inwhich the reflector has six-fold rotational symmetry and a circularprofile.

FIG. 28 shows three variants of a reflector, showing the effect ofvarying the parameter controlling the positioning of the riser in thereflector.

FIGS. 29 and 30: Embodiment for Close-Packed Arrays

FIG. 29 shows an embodiment of the invention in which the reflector ispart of a module providing two-axis pivoting and designed to be packedclosely with other such modules in an array.

FIG. 30 shows several views and cross-sections of the reflector.

FIGS. 31 and 32: Fixed-Geometry Arrays

FIG. 31 shows an embodiment of the invention in which the reflector isthe union of a tiling of identical cells having square apertures.

FIG. 32 shows an embodiment of the invention in which the reflector isthe union of a tiling of identical cells having hexagonal apertures.

FIG. 33: Reflector Having Reduced Surface Area

FIG. 33 shows a reflector of an embodiment of the invention in which therisers in which receivers are located are flanked by paraboloid mirrorsinstead of risers.

DETAILED DESCRIPTION OF THE INVENTION

The invention is a novel method of capturing and focusing light whoseapplications including the concentration of solar energy for conversioninto usable forms of energy such as electricity and/or heated fluids.The invention combines multiple parabolic mirrored surfaces and at leastone receiver, such as an energy-conversion or imaging device, situatedalong focal lines, arcs, or points of said reflective surfaces, in arigid element mounted to tilt on one or more axes so as to maintain itin an orientation relative to directional incident light such that saidlight remains focused on said receiver(s).

Seen from the direction of incident light focused by an element, theelement presents a set of contiguous faces while hiding the receiversfrom direct view in risers situated between said faces and orientedparallel to the direction of view. Each receiver straddles the focalline, arc, or point of one or more of the parabolic mirrors on theopposite side of the element, and the focal line, arc, or point of eachparabolic face is straddled by a receiver.

The cross-section of a reflector roughly approximates a ‘V’ making anangle of 90 degrees. Stated differently, a typical reflector of theinvention can be partitioned into one or more pairs of opposite sectionssymmetrically disposed around the reflector's axis of symmetry, wherethe average surface normal of a section is roughly perpendicular to thatof its opposite section. The invention allows the creation of reflectorswhose cross-sections can be circumscribed by a half disk having adiameter equaling the reflector's aperture.

Because of their compact geometry, multiple elements can be mountedadjacent to each other in an array to cover an area such thatessentially all of the directional light falling in the aperture of thatarea will fall on reflective surfaces and be concentrated onto thereceivers modulo some losses around the edges of the aperture thatincrease as the direction of incident light becomes more oblique. Theefficiency with which such an array captures directional light, combinedwith the shallow form-factors made possible by the shapes of theelements, makes the invention ideally suited for concentrating solarpanels using a small fraction of the photosensitive materials requiredby conventional solar panels.

The reflectors in applications of the invention to solar energycollection include both forms that require only one axis of angularmovement to maintain the focus of incident light upon the receivers asthe sun moves, and forms that require two such axes. The followingdescription and the accompanying figures starts with a brief overview ofrepresentative types of embodiments of both types, then examines methodsof generating reflector profiles of the invention's simplest form, thendiscloses several one-axis embodiments of the invention in the form ofcomplete solar concentrating systems, and finally examines two-axismethods and embodiments of the invention.

FIG. 1 shows the reflectors of six embodiments of the invention, labeledA through E. For clarity, only selected axes of revolution and theportions of receivers visible on the reflectors' undersides are labeled.The receivers are of three types: strip-like and linear 1, strip-likeand arcuate 3, and spot-like 5. The axes of revolution define mirrorfaces swept out by parabolas and are of two types: those offset fromtheir respective parabolas' axes 4, and those coinciding with theirrespective parabolas' axes 6.

The six exemplary reflectors have different characteristics. ReflectorsA and B are suitable for one-axis tracking, where A embodies thesimplest form of the invention, having four mirrors and two receiverssymmetrically disposed, and B is a variation which multiplies the numberof receivers and intervening mirrors. Reflectors C through F requiretwo-axis tracking to keep sunlight in focus. The set of six exhibitsthree types of concentration geometries using three types of parabolicmirrors, each of which focuses directional light differently. ReflectorsA and B use parabolic cylinder mirrors to focus light onto linear bandsand require elongate receivers running the length of the reflector.Reflectors C and D use paraboloid-like mirrors to focus light ontoarcuate bands and require curved elongate receivers of some length.Reflectors E and F use paraboloid mirrors to focus light onto spots andrequire only very small receivers. Unlike the simple paraboloids ofrevolution used by reflectors E and F, reflectors C and D use types ofparaboloids in which the axis of revolution is offset from the focus ofthe generating parabola.

The different types of reflectors shown in FIG. 1 each provide differentadvantages. Reflectors like types A and B, are attractive candidates forCPV panels because of their geometric simplicity, and need for onlyone-axis tracking, and potential ease of manufacturing. Reflectors liketypes E and F are attractive because their provision of very highconcentration ratios reduces the required photovoltaic materials to apoint that enables the economic use of the most efficient photovoltaiccells available. Reflectors like types C and D are attractive becausethe light they direct to any point on a receiver is essentiallyco-planar and therefore has a lower average angle of incidence than thatprovided by reflectors having parabolic cylinder or simple paraboloidmirrors.

The following five sections describe in detail two complete embodimentsusing one-axis reflectors. The first embodiment, a rooftop photovoltaicpanel, is based on the simplest of a family of forms contemplated by theinvention, in which the two facing sides of a slat's upper face aresymmetric and are each made up of two parabolic cylinders joined by aplanar strip. The second embodiment, a multi-modal photovoltaic window,uses a more complex form in which the slat's facing sides are notsymmetric, and are each made up of four parabolic cylinders and twoplanar strips.

In addition to the two complete embodiments, portions of otherembodiments are presented to illustrate additional features, some ofwhich can be used interchangeably in a variety of arrangementsconstituting complete devices.

Method Overview: FIGS. 2 through 5

This section describes the geometric construction of the simplest formof the invention defined above, its relationship to prior art, andmethods of determining the optimal form of the invention based onperformance criteria such as aperture efficiency and trackingrange-of-motion. The embodiment shown in FIGS. 6 through 13, is based onthese methods.

FIG. 2 shows cross-sections of a slat 12 and portions of its adjacentneighbors 14 and 16 in an array of slats rotated about their respectivepivot axes 18 20 degrees away from the array's normal direction 40 tomatch the declination of incident light, which is indicated by theparallel dotted lines 10. Each slat has the four parabolic cylindermirrors, 22, 24, 26, and 28 and the risers 32 and 34 whose planar facesare equipped with the photovoltaic strips 60, where the mirrors 22 and24 are shaped to reflect directional light onto the strip on the riser34 and the mirrors 26 and 28 are shaped to reflect directional lightonto the strip on the riser 32.

The dashed circular arc 42 indicates the path swept out by the slat'sedges 36 and 38 as the slat pivots about its axis 18, and represents theclearance profile of the slat. The dashed pairs of lines 46 indicate thespacing between the clearance profiles of adjacent slats.

The slats whose cross-sections are pictured in FIG. 2 lie completelywithin their said clearance profiles. When the slats in an array areoriented so as to capture light whose direction projected onto the planeof the slats' cross-section is perpendicular to the array's horizontalaxis, the slats' edges nearly touch, allowing them to capture nearly allof the light. When the same slats are oriented to as to capture lightwhose direction projected onto the plane of the slats' cross-section isrotated from said perpendicular direction by an angle whose magnitude isbetween a few degrees more than zero and a few degrees less than 90, theslats will partially shade each other, but will continue to captureessentially all of the light entering the array's aperture, with theexception of some of the light falling on the extreme edges of thearray.

The slats whose cross-sections are pictured in FIG. 2 can rotate tofully 90 degrees to either side of the array's normal direction, and cando so regardless of the angular positions of their neighboring slats.Because the invention allows the design of slat arrays with optimal(dense) spacing in which even unsynchronized slats do not collide overthe entire range of rotation required by practical applications, itmakes feasible the design the slats as mechanically independent moduleshaving their own tilt drive mechanisms. In a panel having such a modulardesign, the malfunction of one slat would not significantly impact theoperation of the remaining slats in the panel, and would only marginallyaffect the panel's power output.

FIG. 3 shows cross-sections and representative light paths of sixmethods of optical concentration using one-axis reflectors, for thepurpose of reviewing prior art. FIGS. 3A through E illustrate theoptical geometries of five representative examples of prior art and FIG.3F illustrates that of the simplest form of the invention. This reviewis restricted to examples of primarily reflective optics that, like theinvention, locate receivers 64 on surfaces that are adjacent to mirroredsurfaces.

FIGS. A through E summarize the optical geometries of concentratorsdisclosed in (A) U.S. Pat. No. 4,088,121, (B) U.S. patent applicationSer. Nos. 11/654,131 and 11/654,256, (C) U.S. Pat. No. 4,222,368 andU.S. patent application Ser. No. 12/156,189, (D) U.S. Pat. No.6,276,359, and (E) U.S. patent application Ser. No. 12/124,124. ExamplesA and B reflect light downward from symmetrically disposed mirrors toreceivers located at the bottoms of trough-like structures, whereexample B refracts the central column of light to achieve a more compactunit; example C reflects light from a mirror to a facing receiver nearthe top of a slat-like structure; and examples D and E reflect lightupward from mirrors to facing receivers located at the top edges of themirrors in trough-like structures. In contrast to all of these, exampleC reflects light downward and upward from mirrors to facing receiversthat are situated between mirrors.

Several advantages of the invention over these examples of prior artbecome evident when considering their suitability for the application ofCPV panels, which calls for the elements to be individually pivotablymounted within an array of closely packed elements. As can be seen withreference to FIGS. 2 and 4, the simplest form of the invention excels inthe two performance attributes of aperture efficiency and range ofangular motion by providing reflectors that, within such an array, canbe moved through large ranges of angular motion while providingcontinuous coverage of the array's aperture. This is because theinvention enables the creation of reflectors each having an aperturewhose width matches the diameter of a circle circumscribing thereflector's cross-section. None of the prior art examples have thisproperty. Examples A and B impose a trade-off between range of angularmotion and aperture efficiency because of the reflector depth, which isless severe in example B. Example C suffers from a significantlyrestricted range of angular motion for all embodiments that providecontinuous aperture coverage. Examples D and E impose a significantpenalty in aperture efficiency because of the additional clearancerequirements of the receivers mounted at outside top edges of thereflectors. In contrast to these designs, the exemplary embodiment has ashallow V-shaped trough that captures virtually all of the light fallingon it, In contrast to these, the concentrator design depicted in FIG. 3Ehas a V-shaped profile that is nearly twice as wide as it is tall so itcan pivot over a wide angular range within close-packed arrays of suchconcentrators, positions the receivers where they don't create clearancedemands, and reflects light to the receivers with low and balancedangles of incidence.

FIG. 4 illustrates a method for deriving the profiles of reflectors suchas are shown in FIGS. 2 through 16. Although this method is illustratedfor the simplest form of the invention as exemplified in FIGS. 2 through16, persons skilled in the art will be able to develop generalizationsof it suitable for variations contemplated by the invention. The methodhas three input parameters: xC, which is equal to or close to 0.5, xD,which is a typically between 0.01 and 0.03, and xE, which is typicallybetween 0.08 and 0.4. The method starts by defining a parabola 70 as theset of points equidistant from the focus 68 and the directorix 76. Theparabola, whose vertex is at the origin 66, is also the solution to theequation y=x², given the coordinate system scaled such that the focus 68is displaced from the origin along the Y axis by 0.25 units. Next, the xcoordinates xA1, xA2, xB1, and xB2, indicated in the drawing by dashedvertical lines, are computed from said input parameters, as follows:xA2=xC−xD; xB1=xC+xD; xB2=xB1+xE; xA1=xA2*0.5; xM1=xC/2; andxM2=xB2*xC/xB1. These four computed x coordinates are used to define twospans of the parabola 70: a lower span 72, whose ordinates range fromxA1 to xA2; and an upper span 74, whose coordinates range from xB1 toxB2. Next, two new parabolas, and spans thereof, are defined by scalingparabola 70 about its focus 68, such that the three parabolas share thesame focus: a slightly larger parabola 90 and span thereof 92 generatedby scaling parabola 70 and its lower span 72 by the factor xB1/xC, and aslightly smaller parabola 80 and span thereof 84 generated by scalingparabola 70 and its upper span 74 by the factor xA2/xC. Given thisconstruction, the rightmost point 96 of the new lower span and leftmostpoint 86 of the new upper span share the same x coordinate of xC. Theselower and upper spans are joined by a vertical segment connecting points96 and 86.

The curve generated by this construction is then combined with itsreflection through the vertical line whose x coordinate is xM1 to createthe symmetric upper profile of the slat's cross-section. The slat'slower profile is a curve that lies entirely below the upper profile andrelatively close to it so as not to expand the clearance profile of theslat. For a slat produced by forming flat stock such as aluminum plate,the lower profile will be approximately a parallel curve to the upperprofile at a distance matching the thickness of the stock. A bevel iscut under the slat's outermost top edges to eliminate any overshoot ofthe slat's body beyond the profile of its upper, reflective surfaces.

FIG. 4B shows the complete profile of a slat generated using thismethod. The focal point 68 lying on the riser on the left half of theslat is shared by both mirror-defining parabolas on the right half ofthe slat: the larger parabola 90 defining the lower mirror 92 and thesmaller parabola 80 defining the upper mirror 84. The same relationshipholds between the focal point lying on the riser on the right half ofthe slat and the parabolas defining the mirrors on the left half of theslat.

FIG. 5 illustrates the effect of varying the two design parameters xDand xE defined in the shape derivation described with reference to FIG.4, with the values of xD increasing from the page's left to right, andthe values of xE increasing from the page's top to bottom.

For each of the cases A through D, the dashed arc 42 indicates the pathswept out by the slat's edge 38 as the slat is moved through its rangeof angular motion, and the dashed arc 44 indicates the path swept out bythe slat's normal vector 20. That range of motion is constrained by therequirement that the slat not protrude outside of its zone in the arrayindicated by the vertical lines 50. Given a wall whose left profilecorresponds to the line 50, rotation of the slat in cases C through Fcauses it to collide with the wall at the points 54, where the lines 52,indicating a rotated image of line 50 about the pivot point 18, meetsthe outside surface of the slat. In cases A and B the slat can berotated through 360 degrees without experiencing such a collision.

FIG. 5 shows that certain ranges of values for xD and xE produce slatprofiles with better ranges of angular motion. Increasing the value ofxD—the parameter that determines the height of the verticalrisers—increases the portions of the slat profiles that extend outsideof the clearance profiles of the slats' edges, decreasing the slats'range of motion. Likewise, larger values of xE—the parameter thatdetermines the relative widths of the upper parabolic mirrors 22 and28—increases the portions of the slat profiles that extend outside ofthe clearance profiles of the slats' edges, also decreasing the slats'range of motion. Of the six different profiles shown, A and B have theadvantage that they may be rotated through the entire 180-degree rangeof useful angles without the possibility of colliding with a neighboringslat. Of these, profile B is generally more desirable for at least threereasons: first, it affords more space in the region 58 immediatelybehind the receiver and within the circular clearance profilecorresponding to arc 42 for mounting heat sinks, fluid conductors,electronics or other such equipment; second, it reflects light to thereceivers with a lower average angle of incidence, improving theperformance of the receiver; and third, it positions the receivers wherethey are in closer average proximity to the mass of the slat's body,providing better thermal performance for embodiments that use the slatas a heat sink. Likewise, profile E is generally preferable to D and Ffor the same reasons: in addition to having a much larger range ofmotion than either, it has the aforementioned benefits of the placementof the risers close to the midpoints of the slat's respective sides.

As FIG. 5 shows, the simplest form of the invention benefits from theplacement of the risers close to half way between the bottom vertex andtop edges of the slat. Persons skilled in the art will appreciate thatmore complex forms of the invention are guided by similar design rules.For example, when the number of risers is multiplied, it is beneficialto space them at relatively equal intervals.

Rooftop Concentrating Photovoltaic Panel: FIGS. 6 through 13

The first complete embodiment of the invention, a concentratingphotovoltaic panel suitable for installation on rooftops, is describedwith reference to FIGS. 6 through 13. FIG. 6 is an isometric view of apanel in which the panel's 34 slats are visible beneath the enclosure'stransparent glazing. The detail view occupying the figure's upperportion shows the ends of five slats articulated with the enclosure'sside wall 212 via the pivotal mounting of the slats' axle pegs 120 inthe slat axle sockets 214 integral to said side wall. Said sockets areopen at the top so that the slats can be removed by pulling them upwardwith a gentle force. In this view, only one of each slat's photovoltaicstrips 160 is visible.

FIG. 7 shows views of a single slat. FIGS. 7A and B show assembled andexploded isometric views of the slat with most of the slat's lengthbetween its two ends omitted, and FIG. 7C shows the cross-sectionindicated in FIG. 7A. The slat comprises the slat body 112, having themirrored surfaces 22, 24, 26, and 28; the two end caps 116 and 118; thephotovoltaic strips 160, and the flat electrical cable 176 andintegrated connector 178 Each of said end caps has a pair of channels124 shaped to accept and end of the slat body so as to rigidly mate withit, and an axle peg 120 having a co-axial cavity 122 that accepts theflexible axle spacer 130. The end cap 118 differs from the end cap 116in that the semi-circular portion of the former's perimeter is gearedwith helical teeth so as to mesh with a worm of the wormed shaft shownin FIGS. 8,9, and 11. Said slat body has the notches 114 at both of itsends. At the end mating with the geared end cap 118, the notch allowsthe end cap's gear teeth to have the necessary depth. At the end matingwith the end cap 116 the notch provides an opening for the passage ofthe connector 178 from the upper side of the slat body to its undersidewhile also securing it.

FIG. 8 shows the panel in a state of partial dis-assembly. FIG. 8A showsthe removable cover, comprising the transparent glazing 204 and thecover frame 206, lifted up, a single slat suspended above the panel, and20 of the panel's full complement of 34 slats installed. FIG. 8B is adetail view showing portions of the drive mechanism, including theshaft-mounting gearmotor 242 and a span of the worm shaft 244 which itmounts, said shaft comprising the keyed axle rod 246 and the worms 248rigidly mounted thereupon with a spacing matching that of the slats. Theshaft support blocks 250, which snap into the channel 216 in the sidewall 212, maintain said worm shaft in a precise position throughout itslength. The back wall 224, formed of a thin heat-conductive material,has the corrugations 226 spanning the panel's long (height) dimensionand most of the panel's shorter (width) dimension, being flat near eachof the panel's sides to accommodate drive and electrical components. Thecorrugations serve to stiffen the back wall and provide increasedsurface area for the dissipation of heat. FIG. 8C is a detail viewshowing portions of several slats, which are oriented in the panel'snormal direction. Broad portions of the parabolic cylinders 26 and 28are visible, as are narrow portions of the parabolic cylinders 24.

FIG. 9 shows the underside of the corner of the panel containing thegearmotor 242, where a portion of the back wall 224 has been cut away toreveal portions of the drive mechanism. The worm gear teeth of the slatend caps 118 are meshed with the worms 248.

FIG. 10 shows the underside of the corner of the panel directly oppositethat shown in FIG. 9, where a portion of the back wall 224 has been cutaway to reveal portions of the panel's electronics. The electronicsmodule 262, which contains a microcontroller, has three electricalcables emerging from it: the motor cable 266, which supplies power tothe gearmotor 242; the external power cable 264, which passes through ahole in the back wall sealed by grommet 230; and the branched ribboncable 270, which provides individual conductive paths between each slatand said electronics module. Said ribbon cable has a branch 272 for eachof the slats, each branch comprising a flexible strand and a connectorthat mates with the slat connector 178 to provide three conductive pathsfrom the slat to said electronics module. The electronics of the panelis described in more detail below with reference to FIG. 13.

FIG. 11 shows an exploded view of the panel, in which the components ofthe panel enclosure are separated. The panel's cover, shown in the upperportion of the page, has its four frame components 206 pulled away fromthe transparent glazing 204. The slats, shown in the middle portion ofthe page, are oriented in the panel's normal direction, and are spacedas they would be when installed in the panel. The parts of the panelenclosure other than the cover are shown in the bottom portion of thepage. All of the numbered parts are identified above in the descriptionsof FIGS. 6 through 10, with the exception of the shaft axle sockets 252,which fit into recesses in the top and bottom frame walls 222 and 220,and rotatably mount the worm shaft 244 at its two ends.

FIG. 12 shows details of the photovoltaic strips 160 that convert lightenergy focused thereupon into electrical energy. Because the strips havesmall-scale structure, FIG. 12 provides two levels of magnification anda cross-section of portions of a strip. FIG. 12A shows the end of a slatin which approximately one thirty-second of the length of one of theslat's two strips is visible. FIGS. 12B and C show two magnifications ofthat strip, and FIG. 12D shows a cross-section through the strip and theslat body on which it is mounted. As illustrated in FIG. 13, describedbelow, the photovoltaic cells in a strip are arranged in clusters of tencells wired in series where said clusters are wired in parallel to formthe circuit of a single strip. Referring to FIG. 12B, which shows theentirety of a series cluster, the series clusters are wired toconductive rails that run the length of the strip and flank the cells:the positive rail 164 and negative rail 166 running along the bottom andtop of the strip, respectively. The negative and positive poles of eachseries cluster are connected to the positive and negative railsindividually through the conducting pads 184 and 186, respectively.Referring to FIG. 12C, which shows a single cell and portions of itsneighbors, a cell's back contact 174 covers the cells underside and itsfront contact 172 has an l-shape that overlaps a portion of the backcontact of the cell to its left. The conductive pad 182 provides aconductive path from the front contact of one cell to the back contactof its neighbor, providing a series connection between the two cells.

The electrical structure of the photovoltaic strips simultaneouslysatisfies two important design requirements: delivering electricaloutput from the slats whose electromotive force is in the desirablerange of five to ten volts, and avoiding significant losses due toshading of portions of a slat. The latter requirement arises from thefact that during normal operation in full sunlight, portions of the endsof the photovoltaic strips will be shaded for significant periods oftime because of shadows cast by the slat's end caps and portions of theenclosure. Whereas the performance of a series circuit of photovoltaiccomponents is greatly degraded by the shading of any one component, theperformance of a parallel circuit of many photovoltaic components isonly minimally impacted by the shading of one component. Because theseries clusters of the present embodiment arc very short relative to thestrip's length, the shading of the slats' ends will have minimal impacton its electrical output.

FIG. 13 is an electrical schematic of the panel showing details of theslat's electrical circuits. The figure's main view depicts theenclosure's electrical components and those within portions of threeslats, each of which has upper and lower rows of componentscorresponding to the slat's two photovoltaic strips. The detail view,located above the main view, shows two series clusters of cells in theirentirety, one belonging to each of the slat's two strips. As describedabove with reference to FIG. 12, each cluster's anode is connected tothe rail 164 and each cluster's cathode is connected to the rail 166. Inthe present embodiment, each strip has 78 series clusters. The branchedcable 270, whose branches 272 connect to the slats via the connectors178, has three dedicated conductors for each slat: a common cathodeconductor, and one anode conductor for each of the slat's two strips.This electrical architecture satisfies several important designrequirements. First, it allows the microcontroller 262 to monitor thedifference between the outputs of each slat's two strips for the purposeof inferring the direction in which the slats' normal axes are displacedfrom the direction of incident light. Second, it allows saidmicrocontroller to monitor the individual performance of each slat so asto flag slats that require service or replacement. Third, it allows thecircuits of the individual slats to be isolated in case of an electricalmalfunction such as a short circuit.

Prototype Concentrating Photovoltaic Panel: FIG. 14

A single slat of a panel embodiment of the invention is shown in FIG.14. Developed for a testbed system, this embodiment is less advantageousthan the rooftop panel described above in several respects, includingmanufacturing simplicity and form-factor desirability. However, itillustrates several methods relating to the invention that may bedesirable in certain instances.

FIGS. 14A and B show assembled and exploded isometric views of the slat,and FIG. 14C shows the cross-section indicated in FIG. 14A. In contrastto the first embodiment, whose slats have single-piece bodies, thepresent embodiment has slats whose bodies are assembled from severalparts. The transparent end plates 328 have channels 324 shaped to acceptthe various parts comprising the slat body: the two upper mirrorsegments 312, the lower mirror segment 314, the heat sinks 316, and theglass bar 318. The end plates are held in compression relative to eachother by the tensioner rods 320, thereby holding all of the above partsof the slat body in precise, rigid positions between the end plates. Thecompression forces may be borne entirely by the said glass rod and heatsinks thereby avoiding possible distortion of the mirror segments due tocompressive forces. The photovoltaic cell assemblies 360 are mountedupon said heat sinks.

In contrast to the first embodiment, each slat of the present embodimenthas a dedicated drive system to maintain alignment of the slat's opticalaxis with the direction of incident light. The drive system comprises,in part, the gearmotor 354, the worm 356 axially mounted thereupon, theworm gear 358 that meshes with said worm, the wheel 360 attached to andcoaxial with said worm gear, the axle rod 350 and axle shaft 352mounting said wheel and worm gear for rotation, the sliding innerhousing 344 mounting said axle rod and gearmotor, the fixed outerhousing 342 pressure fit into channels in the end plate 328 and slidablymounting said inner housing, and the spring 346 that applies and outwardpressure to said inner housing and its contents. The axle bolts 330 passthrough holes in said end plates and securely fasten to the walls of thepanel enclosure (not shown) thereby pivotably mounting the slat. Thedrive wheel 360 engages a grip on the same enclosure's facing wall and,driven by said gearmotor, effects angular movement of the slat relativeto the enclosure.

Unlike the slats of the first embodiment, the diameter of clearanceprofile of that shown in FIG. 13 is larger than its aperture width,where the latter equals the diameter of the arc 302 swept out by the outmirror edges. Therefore such slats arranged in a close-packed arraycannot tilt to 90 degrees away from the array's normal direction withoutcolliding. However, owing to the shape of the slat, whose portions lyingoutside of the arc 302 are restricted to the slat's lower angularpotions, such slats in a close-packed array are able to rotate up to 65degrees before colliding, providing adjacent slats are tilting toroughly the same degree.

Asymmetric Slat Profiles: FIGS. 15 and 16

FIGS. 15 and 16 show the profiles of slats in embodiments of theinvention having different optical geometries than the simplest form, asshown in FIGS. 2 through 14. They show variations of one-axis reflectorsthat are less symmetric and more complex than the simplest form.

FIG. 15 shows a cross-section through a slat whose geometry is similarto that of the first embodiment, but which has a secondary mirror inplace of one of the two receivers. The secondary mirror 430 is aparabolic cylinder whose defining parabola's axis is horizontal andperpendicular to those of the primary mirrors 422, 424, 426, and 428.Like the single receiver 60 mounted on the riser 432 between the primarymirrors 426 and 428, the vertical orientation of the secondary mirror430 between the primary mirrors 422 and 424 assures that it is hiddenfrom the parallel directional light 10 when the slat is oriented. Thesecondary mirror 430 is positioned and shaped such that it capturesdirectional light reflected by primary mirrors 426 and 428 and reflectsit a second time to focus it on the receiver 60. Thus, an oriented slatfocuses all if its captured incident light on the receiver, combiningthe light reflected directly from the mirrors 422 and 424 with the lightreflected indirectly from the mirrors 426 and 428 via the secondarymirror 430.

The secondary mirror 430 has a parabolic profile whose focus 436 isslightly to the left of the shared focal point 434 of the mirrors 426and 428. As a result, the light reflected by the secondary mirrorconverges before reaching the receiver, rather than remaining parallelas would be the case if the points 434 and 436 were coincident.

A slat could employ one or more secondary mirrors, and those mirrorswould not need to have parabolic profiles in order to focus reflectedlight on the receivers. For example, it is possible to design a slatgeometry with secondary mirrors having convex elliptical profiles.

FIG. 16 shows a cross-section of a slat having three mirrors and tworeceivers on each of its facing sides in an asymmetric shape thatroughly approximates a chevron making a 90-degree angle. In thisembodiment, a properly oriented slat directs incident directional light10 as follows. Light falling on the mirrors 522 and 523 is reflected tothe receiver on the riser 538, light falling on the mirror 524 isreflected to the receiver on the riser 536, light falling on the mirrors525 and 526 is reflected to the receiver on the riser 534, and lightfalling on the mirror 527 is reflected to the receiver on the riser 532.

Slat geometries having larger numbers of mirrors and receivers may haveparticular advantages in some applications of the invention. Compared toa single receiver on each transverse half of the slat, a series ofsmaller evenly spaced receivers would increase the ratio of conductivematerial to photosensitive material in the immediate vicinities of thefocal lines, providing more effective conductive heat dissipation for agiven size of slat.

Window with Retracting Photovoltaic Concentrator: FIGS. 17 through 26

The second complete embodiment of the invention, a multi-function windowwith a retracting photovoltaic concentrator, is described with referenceto FIGS. 17 through 26. FIG. 17 shows a trimetric view of the windowwhose concentrator is deployed and its slats tracking the sun. FIG. 18shows a trimetric view of the same window whose concentrator isretracted, providing an unobstructed view through the window. FIGS. 17and 18 show the embodiment in two of its three modes. In a third mode,the deployed concentrator shutters closed to prevent radiative heat lossbetween the interior and exterior space separated by the window.

FIG. 19 illustrates the optical geometry of the slat used in the windowembodiment, showing the cross-section of a slat whose normal direction20 is aligned with the vertical axis of the page, and the paths taken byrays of directional light 10 falling on the slat. The geometry issimilar to that of the slat illustrated in FIG. 16 but differs in havingeight mirrors instead of six, and thereby roughly equalizes the quantityof light convergent on each of the four receivers. In this embodiment, aproperly oriented slat directs incident directional light 10 as follows.Light falling on the mirrors 622 and 623 is reflected to the receiver onthe riser 638, light falling on the mirror 624 and 625 is reflected tothe receiver on the riser 636, light falling on the mirrors 626 and 627is reflected to the receiver on the riser 634, and light falling on themirrors 628 and 629 is reflected to the receiver on the riser 632. Thehorizontal span of each pair of mirrors sharing the same target receiverare approximately equal, as can be seen by the partitioning of incidentlight into bands according to the receiver it is directed to. Thus,incident light in the bands 602, 604, 606, and 608 is reflected to thereceivers at the risers 638, 636, 634, and 632, respectively.

The line 644 spanning the slat's width is the profile of the upper faceof the slat's end plates 640, which are seen more clearly in FIG. 21.The end plates are tilted slightly about the slat's pivot axis 18 withrespect to the plane perpendicular to the slat's normal direction 20 toenable adjacent slats to stack compactly, where every other slat isrotated 180 degrees about an axis perpendicular to the plane of its endplates.

FIG. 20 shows cross-sections through a pair of adjacent slat assembliesin each of the embodiment's three modes: energy capture (A), shuttered(B), and retracted (C). The flat cable 652, gearmotor 674, and tiltwheel 676 are visible only in the lower slat assembly of each pairbecause only one end of each slat is equipped with these components, andthat end is seen only in the lower slat assembly, which is flippedrelative to the upper assembly. In the energy capture mode shown in FIG.20A, the normal directions 20 of the slats are parallel because both arealigned with the direction of incident light. In the shuttering modeshown in FIG. 20B, the end plates of the slats are co-planar, and theedges of the slats touch, creating a barrier to convective and radiativeheat loss. In the retracted mode shown in FIG. 20C, the end plates ofthe slats are parallel and stacked against each other, and the slatprofiles seen in cross-section nest compactly.

FIG. 21 shows views of a single slat and its associated hangerassemblies. FIGS. 21A and B show assembled and exploded isometric viewsof a slat and said assemblies with most of the slat's length between itstwo ends omitted. The slat assembly consists of various small componentsand the single-piece slat body, whose main features are the elongateV-shaped trough forming the parabolic cylinders and risers describedwith reference to FIG. 19, the sloping ends of said trough, the sculptedend plates 640, and the axle pegs 642. The slat assembly's componentsinclude: the photovoltaic strips 650; the branching flat cable 652; theelectronics module 660 incorporating the conductors 662, themicroprocessor 664, and the axle electrical contacts 666; and the tiltdrive assembly, comprising the spring bracket 672, the tilt gearmotor674, and the tilt wheel 676.

The hanger assembly, a pair of which pivotably mounts the slat assemblyat the latter's two ends, comprises: the support bracket 682, having theaxle socket 684, the strap grooves 686 and the cable eyelets 688; theshape-retaining strap 690, having the cut-outs 692; and the strap anchorshims 696.

FIG. 22 shows three views of an assembly consisting of four adjacentslats and the hanger assemblies connecting them, where the hanger straps690 arc in their relaxed position, bent slightly outward. Because thestraps deflect away from the tilt wheels 696, the wheels cannot getfraction except when the slats are facing directly upward. Consequently,the slats automatically align facing upward.

FIG. 23 shows two views of the assembly shown in FIG. 22 in whichtension applied to the straps 690 pulls them taught, such that the tiltwheels 676 engage them regardless of the tilts of their slats, and saidtilt wheels are able to drive their respective slats to any tilt angle.In the figure, all four slats are facing directly to the right, withtheir normal axes aligned with the array's normal axis.

FIGS. 24 and 25 show cross-sections through the upper portion of theembodiment. FIG. 24 shows the concentrator system deployed in its energycapture mode, and FIG. 25 shows the concentrator in its retractedposition. In both figures, the section plane is above window'smid-plane. Features of concentrator system labeled in thesecross-sections are described in other figures, such as FIG. 26,described below. Features of the enclosure include the parallel panes ofglazing 782, the top frame component 776, with its top channel 778providing a path for wires, and the side frame component 790.

FIG. 26 shows the retracting concentrator system revealed by hiding thewindow's exterior frame and glazing, where said system is in a state ofpartial retraction. The main view of the system is supplemented by fourdetail views. The retraction mechanism uses a single continuous loop ofcable 722 to deploy and retract the array of slats, where the cable runsalong the sides and top of the system, is guided by the four bottompulleys 730 the four cross pulleys 734 and the four corner pulleys 738,and is moved by the drive mechanism magnified in the detail view in thefigure's upper left portion.

The drive mechanism, which functions to move the cable in eitherdirection and to maintain its tension even as it stretches slightly overtime, comprises two assemblies: a drive assembly that is rigidly mountedin the top component of the window frame 776, (seen in cross-section inFIGS. 24 and 25) and a sheave assembly that is slidably mounted in saidcomponent. The drive assembly comprises the drive block 744, thedual-groove spool 748 rotatably mounted on a spindle integral to saidblock, the gearmotor 746 that controls the rotation of said spool. Thesheave assembly comprises the sheave block 752, the dual-grove spool 754rotatably mounted on a spindle integral to said sheave block, and thespring 756 mounted on a process in said block and compressively loadedagainst the drive block 744. The cable 722 makes two circuits throughthe drive mechanism, in each case passing by the first spool, wrappingabout 220 degrees around the second spool, crossing back to the firstspool and wrapping about 220 degrees around it then passing back by thesecond spool. The span of cable that enters the mechanism from the frontleft as seen in the figure engages the lower grooves of the spools andexits to the back right, and the span that enters from the front rightengages the upper groves of the spools and exits to the back left.

The cable 722 has eight spans that run between the bottom pulleys 730and the top pulleys 734 or 738, and these can be divided into the fourouter spans situated just beyond the edges of the folded straps 692 andthe four inner spans that thread the eyelets 688 in the hanger brackets682. Four bead-like nodes 724 are fixed to the cable at each of fourpoints along said inner spans at the same height, and the pair of hangerbrackets mounting the bottom-most slat are each secured to two of thesenodes via their respective eyelets. The routing of the cable is suchthat when the cable moves under the influence of the drive mechanism,said nodes belonging to all four said inner spans move in unison in thesame direction, and maintain the same height, thereby assuring that thefour points holding the bottom-most pair of hanger brackets remainparallel and perpendicular to the window's vertical walls.

To deploy the concentrator, the drive spool 748 rotates in a clockwisedirection as seen in FIG. 26, causing the four nodes 624 and thebottom-most hanger pair secured thereto to descend. As said hangerreaches the reaches its deployed position, the hanger straps 690 becometaught, ensuring equal spacing of the slats, and providing surfaces uponwhich the slats' tilt wheels 676 can gain traction. To retract theconcentrator, the drive spool 748 rotates in a counter-clockwisedirection, causing the four nodes and the bottom-most hanger pair toascend. As the hanger straps relax, they bend outward, causing theslat's tilt wheels to loose traction and the slats to thereby assume anupward-facing position that allows them to stack compactly as thebottom-most slat ascends.

Referring to FIGS. 24 through 26, an electronics module 762 mounted inthe window frame's top component 776 contains a microprocessor and awireless communication device allowing the panel's modes to becontrolled via a hand-held remote-control unit. Said module is equippedwith the electrical cable 764 to send and receive power and data from anexternal electrical system, a wire 766 supplying power to the retractordrive motor, and a pair of electrical cables linking it to electricalconnectors in the pair of electrical cables 768 that plug intoconnectors 706 in the top pair of hanger brackets 702 to provideelectrical connectivity to the slats assemblies. Conductors embedded inthe hanger brackets 702 and 682 and in the hanger straps 690 provideconductive paths from the connectors 706 to each of the slat assembly'saxle electrical contacts 666, and thereby provide circuits between saidelectronics module and the electrical systems of the slats. Thesecircuits are parallel, enabling two conductive paths traversing saidhanger brackets and straps on each side of the concentrator assembly toserve all of the slats, and summing the electrical current delivered bythe slats when operating in their energy-gathering mode. These circuitsalso provide a constant voltage to power the slats' electronics whenthey are in other modes. The same conductive paths are used for two-waycommunication between the electronics module 762 and the slats'microcontrollers 664, using digitally encoded signals that aresuperimposed on and do not interfere with the delivery of analog power.

When the concentrator array is in its energy collecting mode, the slatsindividually track the motion of the sun, each adjusting its tilt angleto bring its normal axis into alignment with the direction of light. Theslat assembly's tilt controller may perform this function autonomously,or may do so with input from the panel's controller. For example, thetilt controller might query the panel controller for its estimate of thecurrent angular position of the sun, move the slat to match that angle,and then repeatedly measure the relative outputs of the receivers on thetwo facing sides of the slat and rotate the slat a small amount in thedirection that moves the side with greater output closer to the sun.

Two-Axis Reflector Examples: FIGS. 27 and 28

FIG. 27 shows an embodiment of the invention in which the reflector hassix-fold rotational symmetry and a circular profile. FIG. 27A is a viewfrom the direction of the reflector's optical axis, and FIG. 27B is across-section through a reflective plane of symmetry of the reflectorthat contains two of the reflector's six focal points.

The reflector 800 depicted in FIG. 27 has six identical sections, eachoccupying a 60-degree wedge and each having two paraboloid mirrors and aphotovoltaic cell situated in a riser straddling the coincident focalpoints of the two paraboloid mirrors in the opposite section of thereflector. In FIG. 1, the six sections have, respectively, the largemirrored paraboloid faces 821, 822, 823, 824, 825, and 826; the smallmirrored paraboloid faces 831, 832, 833, 834, 835, and 836; and thereceivers 841, 842, 843, 844, 845, and 846.

Each receiver is embedded in a riser 802 intervening between a largeparaboloid mirror and a small paraboloid mirror. Said receiver isembedded in the one of the three facets of the riser that faces theopposite section of the reflector, and straddles the common focal pointof the two mirrors in that section. Thus, for example, cell 844straddles focal point 854, and is situated so as to absorb reflectedlight from both the small mirror 831 and the large mirror 821 in theopposing section of the six-fold reflector.

The dotted lines 862 and 864 show how two rays of incident light, eachapproaching the reflector parallel its optical axis, are reflected.Light ray 862 is reflected by paraboloid face 831 to land on receiver844, and light ray 864 is reflected by paraboloid face 821 to land onreceiver 844.

FIG. 28 shows the effect of varying the design parameter that determinesthe positioning of focus-coincident risers between the reflector'svertex and its edges in a reflector embodiment having a circular profileas seen from the direction of its optical axis and fourfold rotationalsymmetry. This design parameter is the same one as that determining theplacement of the riser described with reference to FIG. 5, in this caseapplied to a reflector with paraboloid mirrors. The figure shows threereflectors labeled A, B, and C, as two views of each: a trimetric viewon the left and a cross-sectional view on the right. The dashed circle42 superimposed on each cross-sectional view shows the approximateclearance profile required by the reflectors' edges as the reflector ispivoted about their centroid of the outermost points of the reflector'sedge.

FIG. 28 also shows some of the geometric entities shown in FIG. 4describing the generation of the reflector's shape. The cross-sectionalviews show the lower and upper parabolas 80 and 90 that generate theparaboloid surfaces. The axis 78, corresponding to the Y axis in FIG. 4Aand containing the shared focus 68 and vertices 81 and 91 of theparabolas 80 and 90, is also the axis of revolution of the paraboloidsgenerated by them.

As is the case with the one-axis reflectors examined in FIG. 5, the moredesirable instances of the simple form of paraboloid reflectors shown inFIG. 28 position the risers at an intermediate distance between theupper edges and lower vertex of the reflector, with case B beingpreferable to cases A and C. Case B in FIG. 28 is advantageous for thesame set of reasons that cases B and E in FIG. 5 are. Compared to caseA, case B provides more space behind the focal points 68 for mountingelectronics and heat management equipment associated with the receivers.Unlike case C, case B fits the reflector entirely within the sphericalclearance profile indicated by the dashed circle 42, enabling it to bepivoted about the centroid of its upper edge in an unrestricted range ofmotion within a densely packed array. Compared to cases A and C, case Bprovides a smaller average incidence angle upon its receivers.

Four-Fold Reflector With Profile Suited to Close-Packed Arrays

FIGS. 29 and 30 show an embodiment of the invention in which thereflector has two reflective symmetries and an operational clearanceprofile designed for use in close-packed arrays of reflectors, each suchreflector is allowed to move independently. FIG. 29 shows the reflectorand its associated support and angular positioning apparatus 900, wherethe latter effects movement of the reflector about two perpendicularpivoting axes which intersect at the centroid of the reflector's upperedge. FIG. 30 shows views and sections of the reflector, where FIG. 20Ais an isometric view of the reflector, FIG. 30B is a view down itsoptical axis and FIGS. 30C through E are cross-sections indicated inFIG. 30B.

The reflector 910 has four sections which are identical to, or mirrorimages of, each other, each section occupying one of the four quadrantsdivided by the section lines C and E in FIG. 30B. Each quadrant has aset of paraboloid mirrors that share an optical axis and coincidentfocus 912 situated on a riser 932 on the opposite, facing side thereflector. Only the paraboloid mirrors and receiver in the upper-rightquadrant as seen in FIG. 30B are labeled, and only portions of the riserin the lower-left quadrant are labeled. The two largest paraboloids 922and 925 are separated by a span of the riser 932 that contains thereceiver 960, and five other paraboloids separated by shallower spans ofsaid riser. In order of decreasing scale and increasing height, theparaboloids are 922, 923, 924, 925, 926, 927, and 928. When viewed downthe reflector's normal axis as in FIG. 30B, the paraboloid mirrorsentirely cover the profile of the reflector, where the riser 932separating the mirrors, being parallel to said axis, appears as a set oflines and arcs.

The bounding shapes of the paraboloid faces and the paraboloidscale-factors defining their three-dimensional shapes are selected tofulfill a variety of objectives including: providing risers in positionsand with dimensions suitable for mounting receivers; providingunobstructed paths for light from the paraboloid faces to the receivers;and fitting the entire reflector within a desired clearance profile.

The shape of the reflector shown in FIGS. 29 and 30 was designed to fitin the clearance profile required by modules using the two-axis angularpositioning means described in U.S. provisional patent filing61/200,835. That document describes a one-parameter family of flatshapes each of which, when mounted for two-axis rotation using saidangular positioning means, traces out a three-dimensional shape whoseprojection onto the X-Y plane remains entirely within the original flatshape. Shallow dish-type reflectors whose X-Y profile closelyapproximates such a shape, and whose rim deviates only slightly from theplane parallel to the X-Y plane containing the angular positioner'srotation centroid, can be packed into dense arrays such that neighboringreflectors cannot collide no matter how they move on their respectivemounts.

Fixed-Geometry Arrays: FIGS. 31 and 32

FIGS. 31 and 32 show embodiments of the invention in which the reflectoris composed of an array of identical cells in fixed relationship toone-another, where each cell has the essential features of theinvention's optical geometry. FIG. 31A shows a trimetric view of areflector consisting of nine cells, each of which has eight paraboloidmirrors and four receivers in a configuration having fourfold rotationalsymmetry and four planes of reflective symmetry, and FIG. 31B shows areceiver assembly pair removed from the array. FIG. 32 shows anisometric view of a reflector consisting of sixteen cells, each of whichhas twelve paraboloid mirrors and six receivers in a configurationhaving sixfold rotational symmetry and six planes of reflectivesymmetry. In each of the figures only one representative mirror of therespective embodiment's two types of mirrors is labeled.

In the embodiment shown in FIG. 31, the reflector body 1010 spans thefixed array of nine cells. Each of the four sections of each cell hastwo paraboloid mirrors, the upper mirror 1022 and the lower mirror 1024separated by the riser 1032, where said riser is perforated by arectangular hole filled by the photovoltaic cell 1060 of a receiverassembly. Portions of several such receiver assemblies 1062 arc visibleon the facing sides of the array, and a pair of two such assemblies,such are found in the interior of the array, is shown in FIG. 31B. Acylindrical cavity in the body of the receiver assembly allows passageof the wires 1068 from the photovoltaic cells, and the fins 1064 on saidbody facilitate dissipation of heat.

In the embodiment shown in FIG. 32, the reflector body 1110 spans thefixed array of sixteen cells. Each of the six sections of each cell hastwo paraboloid mirrors, the upper mirror 1122 and the lower mirror 1124separated by the riser 1132, where said riser is perforated by arectangular hole filled by the photovoltaic cell of a receiver assemblylike that described with reference to FIG. 31. This embodiment has theadvantage over the previous one that the average angle of incidence oflight on the receivers is less, whereas the previous embodiment has theadvantage that the reflector exactly covers a rectangular region and cantherefore fill the aperture of a rectangular enclosure.

Reflector Having Reduced Surface Area: FIG. 33

FIG. 33 shows the reflector of an embodiment of the invention in whichthe extent of vertical risers has been reduced by replacing withparaboloid mirrors those riser faces not having receivers. FIG. 33Ashows an isometric view of the reflector, and FIG. 33B shows a view downthe reflector's normal axis. The reflector has six-fold rotationalsymmetry about the axis 1220 in which each of six identical sections hasa plane of reflective symmetry. In FIG. 33, the mirrors and riser ofonly one such section are labeled.

Like embodiments described above, the reflector has a larger paraboloidmirror 1224 below the riser 1232 and a smaller paraboloid mirror 1222above said riser, where both mirrors share the focal point 1212 situatedin the riser of the opposite section of the reflector. Unlikeabove-described embodiments, the present reflector has the paraboloidmirrors 1226 and 1228, which have the focal points 1216 and 1218,respectively, said focal points being in the reflector sections toeither side of the section immediately opposite the section having thosemirrors. Thus, whereas the paraboloid mirrors 1222 and 1224 share theaxis of revolution 1202, the paraboloid mirrors 1226 and 1228 have theaxes of revolution 1206 and 1208, respectively.

FIG. 33A also shows the parabola 1246 whose rotation about the axis 1206generates the paraboloid to which the mirror 1226 belongs, where thepoints 1245 and 1247 bound the portion of said parabola that sweeps outthe smallest portion of the paraboloid covering the said mirror. FIG.33B also shows the paths of representative light rays reflected by eachof the labeled mirrors as dashed lines.

Compared to similar previously described embodiments, the reflectorshown in FIG. 33 requires has less surface area and therefore requiresless material, and has fewer risers and therefore fewer sharp dihedralangles between risers and mirrors. As a result this embodimentpotentially affords greater economy of material and ease of manufacture.It does so at the expense of slightly higher average angles of incidenceon the receivers due to the angular relationship between the mirrors1226 and 1228 and the receivers to which they direct light.

Persons skilled in the art will be able to foresee many modifications tothe described embodiments. For example, the reflector shown in FIG. 33could be modified to have eightfold rotational symmetry, and the mirrorsflanking the risers 1232 in each of the reflector's eight sections couldhave focal points situated in the second instead of the first sectionsto either side of the directly opposite section. The upper paraboloidmirrors 1222 could be eliminated. The reflector could be furthersimplified by eliminating all but the largest paraboloid mirrors 1224,with receivers protruding above those mirrors, but such a modificationwould reduce the reflector's aperture efficiency.

CONCLUSION, RAMIFICATIONS, AND SCOPE OF THE INVENTION

While the above description contains many specifics, these should not beconstrued as limitations on the scope of the invention, but rather asexemplifications of several embodiments thereof. Many other variationsare possible. Accordingly, the scope of the invention should bedetermined not by the embodiments illustrated but by the appended claimsand their legal equivalents.

1-16. (canceled)
 17. A solar concentrating device comprising: aplurality of slats, wherein each of the plurality of slats comprises: atleast one first elongate reflective surface configured to focus incidentsolar radiation onto at least one first receiver; at least one secondelongate reflective surface configured to focus incident solar radiationonto at least one second receiver; and an aperture defined by the atleast one first elongate reflective surface and the at least one secondelongate reflective surface; and a mounting structure mechanicallycoupled to the plurality of slats and configured to retract the array ofslats into a compact stack.
 18. The solar concentrating device of claim17, comprising a solar tracking controller, wherein the plurality ofslats is configured to track incident solar radiation.
 19. The solarconcentrating device of claim 17, comprising a solar trackingcontroller, wherein each of the plurality of slats is configured toindependently track incident solar radiation.
 20. The solarconcentrating device of claim 19, wherein a signal derived from at leastone of the receivers is used as an input to the solar trackingcontroller.
 21. The solar concentrating device of claim 17, wherein theat least one first receiver and the at least one second receivercomprise photovoltaic cells.
 22. The solar concentrating device of claim17, wherein a centerline of the aperture defines a tilt axis, and themounting structure is configured to independently tilt each of theplurality of slats about the tilt axis.
 23. The solar concentratingdevice of claim 17, wherein each of the plurality of slats ischaracterized by a V-shaped cross-section having a width and a depth,wherein the width is about half the depth.
 24. The solar concentratingdevice of claim 17, wherein the at least one first elongate reflectivesurface comprises: an upper first elongate reflective surface and alower first elongate surface; and a first riser connecting the upperfirst elongate surface and the lower first elongate surface.
 25. Thesolar concentrating device of claim 24, wherein the first riser isconfigured to retain the at least one second receiver.
 26. The solarconcentrating device of claim 24, wherein the at least one secondreceiver is mounted on the first riser.
 27. The solar concentratingdevice of claim 17, wherein the at least second elongate reflectivesurface comprises: an upper second elongate reflective surface and alower second elongate surface; and a second riser connecting the uppersecond elongate surface and the lower second elongate surface.
 28. Thesolar concentrating device of claim 27, wherein the second riser isconfigured to retain the at least one first receiver.
 29. The solarconcentrating device of claim 27, wherein the at least one firstreceiver is mounted on the second riser.
 30. The solar concentratingdevice of claim 27, wherein the first elongate reflective surface andthe second elongate reflective surface are connected at an end oppositethe aperture.
 31. The solar concentrating device of claim 17, whereinthe mounting structure is configured as a window-type enclosure.
 32. Thesolar concentrating device of claim 17, wherein the plurality of slatsis configured such that an edge of the aperture of one of the pluralityof slats is configured to abut with an edge of an aperture of anadjacent one of the plurality of slats.
 33. The solar concentratingdevice of claim 17, wherein each of the at least one first receiver andeach the at least one second receiver are strips comprising photovoltaiccells and electrical conductors, wherein: the photovoltaic cells arearranged in clusters, each cluster occupying a short span of the stripand the totality of clusters occupying the length of the strip; theelectrical conductors comprise a negative rail and a positive railextending a length of the strip; the photovoltaic cells in each clusterare electrically connected to each other in series; and each cluster iselectrically connected in parallel to the negative and to the positiverail.
 34. The concentrating device of claim 17, wherein each of theplurality of slats is configured to conduct heat away from the at leastone first receiver and from the at least one second receiver.
 35. Asolar concentrating device comprising: a first elongate reflectivesurface comprising a first upper elongate reflective surface, a firstlower elongate reflective surface and a riser connecting the first upperelongate reflective surface and the first lower elongate reflectivesurface; and a second elongate surface; wherein the first elongatereflective surface and the second elongate reflective surface define anaperture; the first elongate reflective surface is configured to directincident solar radiation onto the second elongate reflective surface;and the second elongate reflective surface is configured to focusincident solar radiation and radiation directed from the first elongatereflective surface to a line focus in the vicinity of the riser.
 36. Thesolar concentrating device of claim 35, wherein one or more photovoltaiccells are disposed in the vicinity of the line focus.
 37. The solarconcentrating device of claim 35, wherein one or more photovoltaic cellsare mounted on the riser.
 38. A solar concentrating device comprising: amounting structure; and a plurality of slats mechanically coupled to themounting structure, wherein each of the plurality of slats comprises: anupper side and a length; a receiver affixed to the upper side andextending along the length; at least one first elongate reflectivesurface, a second elongate reflective surface, and a third elongatereflective surface, wherein each reflective surface is disposed on theuppers side of the slat and is configured such that incident radiationfalling on the first elongate reflective surface is focused directlyonto the receiver, and incident radiation falling on the second elongatereflective surface is focused indirectly onto the receiver by reflectionfrom the third elongate reflective surface.
 39. The solar concentratingdevice of claim 38, wherein: the first elongate reflective surface isadjacent to the third elongate reflective surface; and the secondreflective surface is adjacent to the receiver.